1. Field of the Invention
The invention relates to an X-ray examination apparatus, including an X-ray source for emitting an X-ray beam in order to form an X-ray image of an object, an X-ray detector for detecting the X-ray image and converting it into an electronic image signal, and an exposure control circuit for forming a control signal from the electronic image signal in order to adjust the X-ray source. The invention also relates to a method of controlling, by way of feedback, an X-ray source which irradiates an object, thus forming an X-ray image wherefrom an electronic image signal is formed wherefrom a control signal is derived for controlling the X-ray source.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from German Auslegeschrift DE 26 10 845.
The known X-ray examination apparatus comprises a selector switch for selecting one of six predetermined measuring fields in the X-ray image. The control signal is the mean value of the brightness of the X-ray image within the selected measuring field. It is used in a feedback loop to control the power supply unit of the X-ray source, thus controlling the duration of exposure of a patient to be examined so as to achieve adequate brightness and contrast in the X-ray image and to limit the X-ray dose. In the case of overexposure, in the known X-ray apparatus the control signal is affected by a contribution made by an overexposed area within the measuring field selected in the X-ray image. Overexposure occurs whenever substantially non-attenuated X-rays are incident on the X-ray detector, for example because such X-rays have passed adjacent the patient or through a low-absorption part of the patient. Due to contributions of overexposed areas to the control signal, the exposure control circuit of the known X-ray examination apparatus produces a setting which is detrimental to the image quality in areas of the X-ray image outside the overexposed area. Due to the presence of an overexposed area within the selected measuring field, it may occur, for example that the mean brightness in the measuring field is higher than the brightness in an area of the measuring field in which an anatomical structure which is of importance for the examination is reproduced. In such a case the exposure control circuit produces a control signal whereby the X-ray source is controlled so that the mean brightness in the measuring field is adjusted to a desired value, but the brightness in the area with the anatomic structure will then be lower than the brightness required for a suitable reproduction of this anatomical structure is obtained.